Gene Therapy for Leber's Congenital Amaurosis
Leber's Congenital Amaurosis (LCA) refers to a set of disorders relating to retinal disfunction. These disorders are casued by mutations in a number of different genes, each giving rise to a different type of LCA. The retinal disfunction associated with these disorders generally leads to a severe loss of vision in early infancy and blindness later on in life. Almost 10% of the cases of LCA are caused by a mutation in the gene RPE65, which encodes retinal pigment epithilium-specific 65 kDa protein. This form of LCA, called LCA2, has been the focus of several recent gene therapy treatments using the adeno-associated virus (AAV) (1 ,2 ,3 ). RPE65 Retinal pigment epithelium-specific 65 kDa protein, or RPE65, is encoded by the gene RPE65. This protein is improtant during phototransduction, specif ically in the transformation of all-''trans''-retinyl esters into 11-''cis''-retinal. With a disfunctional RPE65, 11-''cis''-retinal is not made and light cannot be transduced into an electrical signal that the brain will recognize. This can lead to severe visual impairment and blindness, such as in the disease LCA2. Mutations in RPE65 can be inherited and cause close to 10% of all cases of LCA (1 , 2 ). Adeno-Associated Virus Adeno-associated virus (AAV) is a single-stranded DNA virus which is used as a vector for gene therapies, such as LCA2. AAV can successfully inject it's DNA genome into a host cell's nucleus, where the genome integrates into the host's DNA, but cannot reproduce in that cell in the absence of a helper virus, such as adenovirus (4 ). This is an important feature because this means that when AAV is used alone to infect cells, it will not reproduce once it delievers the DNA it is carrying. Another factor which makes AAV a good candidate for use in gene therapy is its integration into the human genome at a very specific site on chromosome 19 (4 ). A specific integration site means there is less chance of random integration into a crucial part of the genome. Random integration could lead to a wide range of side effects and even cell death. Treatement of LCA2 Two different studies from 2008 show the efficacy and safety of using AAV for gene therapy in LCA2''. Human ''RPE65 cDNA was incorporated into a replication defective AAV vector and injected behind the retina of the trial patients (3 in each study). The goals of both of these studies were to determine the safety and efficacy of gene therapy in treating LCA2. Both studies show safe administration of the vector with few to no side effects. The side effects noted by the investigators included detachment of the retina at the site of injection, which resolved spontaneuosly after several days, and a slight increase of neutralizing antibody for AAV in the patient's serum, which subsided and had no lasting effects. To determine the efficacy of the treatment, either the visual acuity of the patient was tested, or the patient was placed in a simple maze, where observations on the patient's ability to navigate the maze were made by the investigators (1 , 3 ). A video outlining the study, from administration of the vector to the results of the terapy, can be seen on the New England Journal of Medicine website here . References 1. [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2829748/ Maguire, A, et al. Saftey and Efficacy of Gene Transfer for Leber's Congenital Amaurosis.'' N Eng J Med''. 2008] 2. [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2839440/ Simonelli, F, et al. Gene Therapy for Leber's Congenital Amaurosis is Safe and Effective Through 1.5 Years After Vector Administration. Mol Ther. 2009.] 3. [http://www.nejm.org/doi/full/10.1056/NEJMoa0802268#t=article Bainbridge, J, et al. Effect of Gene Therapy on Visual Function in Leber's Congenital Amaurosis. N Eng J Med. 2008.] 4. [http://www.ncbi.nlm.nih.gov/pmc/articles/PMC53656/pdf/pnas01031-0173.pdf Kotin, R, et al. Site-specific integration of adeno-associated virus. Proc. Natl. Acad. Sci. 1990.]